PowerShell how to add something on parsed JSON?

If you're using PowerShell 3.0/4.0 you can simplify your conversion using the ConvertFrom-Json cmdlet.

Beyond that, if you have PS or .Net Object Types, the Add-Member cmdlet allows you to add arbitrary properties. The following shows how to add a property based on a Json block:

$json = @"{  "BlockA": {    "BlockB": {      "name": "BlockB",      "value": "Value_B"    }  }}"@$blockcvalue =@"    {    "name":"BlockC",    "value":"ValueC"    }"@$jobj = ConvertFrom-Json -InputObject $json$jobj.BlockA | add-member -Name "BlockC" -value (Convertfrom-Json $blockcvalue) -MemberType NotePropertywrite-host (ConvertTo-Json $jobj)

You get that error because your $json is actually a collection of two objects. One of them is an assembly and the other one is a dictionary. Pipe output from the line that loads assembly to Out-Null to avoid that. Example:

function ConvertFrom-Json([String]$sRawJson) {    [System.Reflection.Assembly]::LoadWithPartialName("System.Web.Extensions") `        | Out-Null    $oJsSerializer = `        New-Object System.Web.Script.Serialization.JavaScriptSerializer    return $oJsSerializer.DeserializeObject($sRawJson)}$sBaseContent = @"{    "BlockA": {        "BlockB": {            "name": "BlockB",            "value": "Value_B"        }    }}"@$sBlockcContent  = @"{    "name": "BlockC",    "value": "Value_C"}"@$jsonBaseObj = ConvertFrom-Json($sBaseContent)$jsonBlockcObj = ConvertFrom-Json($sBlockcContent)$jsonBaseObj.BlockA.Add("BlockC", $jsonBlockcObj)$jsonBaseObj